Probably one of my favorite Pokémon, such as Milotic and Umbreon. Also, when I saw the Red Gyarados following the trainer, that made me think that I will definitely want any Shiny Pokémon I obtain to follow me around lol.
I got the game early and I was planning my shinies to follow me. Turns out it's always the first Pokémon in your party that follows you, and I have not found an option to switch the walking Pokémon
However, I did find out that Pidgey's overworld sprite is seriously so cute I may consider not evolving my shiny Pidgey from Diamond to just let it hop after me <3
Pic to share the Pidgey love
Edit: Oh, and the Geodude one is lame, it doesn't float but just 'hops' on the ground. I get the feeling every Pokémon hops the same I think I'll trade a Voltorb/Spheal over soon to see if it rolls, that would be awesome